Various structural components are used to form a typical aircraft. For example, wing and empennage surfaces of an aircraft typically include stringers that are coupled to skin members on the wing and empennage surfaces that cooperatively provide a desired flexural and torsional stiffness to the wing and empennage surfaces.
Aircraft structures may be formed from composite materials, which are generally reinforced polymer-based materials used in place of metals, particularly in applications in which relatively low weight and high mechanical strength is desired. Accordingly, composite materials are widely used in a variety of commercial and military aircraft, terrestrial vehicles and consumer products. A composite material may include a network of reinforcing fibers that are generally applied in layers, and a polymeric resin that substantially wets the reinforcing fibers to form a binding contact between the resin and the reinforcing fibers. The composite material may then be formed into a structural component by a variety of known forming methods, such as an extrusion process or other forming processes.
In an aircraft, a stringer may be used to transfer bending loads in skin panels, and stiffen the skin panels in order to prevent buckling, for example. The stringers and skin panels may be made of composite materials, such as carbon fiber reinforced plastic (CFRP). A composite stringer may be fabricated from multiple plies of reinforcing fibers.
Often, composite parts, such as composite stringers, include one or more portions having at least some degree of curvature. Composite parts with even a slight curvature are difficult to construct with 0° uniaxial fiber tape, because the plies within the tape are unable to stretch to comply with long aspect ratio contours.
Current methods of forming contoured stringers, such as with fiber tape, generate wrinkles on or in the stringers. For example, draping a composite membrane assembly, which includes multiple layers of plies, along a contoured (that is, curved, non-straight) surface causes the plies to stretch and/or compress. When the plies are forced to stretch, bridging and resin pooling may result. On the other hand, when the plies are forced to shrink, wrinkles may be formed. In both stretching and shrinking situations, inspection and repair costs increase.
A known method of using fiber tape to form or otherwise conform to curved surfaces includes forming 90° cuts in the fiber tape, and overlapping plies to maintain strength. The cuts are 90° (that is, perpendicular) to a 0° direction of the fiber tape. In particular, the cuts are perpendicular to a longitudinal plane of the fiber tape. By forming the cuts and overlapping portions of the tape, however, the fiber tape increases in thickness, weight, and complexity. Further, the overlapped portions form bumps in the fiber tape. Additionally, while the 90° cuts provide a certain amount of flexibility to the fiber tape, they do not overcome the problems of shrinking and compression. Consequently, such a method does is still susceptible to wrinkling.
Overall strength of a composite part decreases with an increase in the number of wrinkles. Indeed, it has been found that wrinkles in a composite part may reduce strength of the part by 80% or more.
When used to form a flat surface, a composite membrane assembly generally lays flat without wrinkling. As the composite membrane assembly is laid up to form a flat surface or folded over a 90° edge (for example, a single curvature shape), there generally is little or no tension or compression in the membrane, and therefore the composite membrane assembly does not wrinkle. However, as noted, when used to form an arcuate, curved surface (for example, a double curvature shape), the composite membrane assembly is influenced by compression and/or tension, and therefore wrinkles.
Thus, a need exists for an improved system and method of forming a composite material that is able to stretch to accommodate curved surfaces without wrinkling, while substantially maintaining strength.